The present invention relates to a molded catalyst for hardening unsaturated fatty acids, comprising an intimate mixture of titanium oxide and finely divided palladium metal. In another aspect, the present invention relates to a process for preparing the molded catalyst.
Fatty acids are an important intermediate product in the chemistry of fats. They are obtained either by the oxidation of paraffins, by oxosynthesis from olefins, by saponification of nitriles or by the decomposition of fats. The latter process is of great importance as it utilizes replaceable raw materials.
Fatty acids from vegetable or animal sources have chain lengths of from 12 to 30 carbon atoms. Most of these fatty acids are unsaturated, with up to 3 double bonds. These double bonds, in particular the triple double bonds, are the cause of the low thermal stability of unsaturated fatty acids and the ease with which they undergo oxidation.
For use of the unsaturated fatty acids in industry, the double bonds must be removed by hydrogenation as completely as possible and at the same time the acid character of the fatty acids must be preserved. Hydrogenation to fatty alcohols is in this case undesirable. The procedure resulting in saturated fatty acids without double bonds in the carbon chain is also known as hardening of the fatty acids.
The quality of hardened fatty acids is determined by the iodine number and the acid number in accordance with the regulations of the Deutsches Arzneibuch, 7th edition, 1968. The iodine number (JZ) is a measure of the proportion of unsaturated fatty acids in fats and the acid number (SZ) is used to determine the proportion of free organic acids in fats.
Naturally occurring fats have iodine numbers ranging from 150 (soya bean oil) to 50 (beef tallow), depending on the degree of saturation. The object of technical hardening of fatty acids from such natural sources is to reduce the iodine number to values below 1 while leaving the acid number as far as possible unchanged to improve the color, odor and resistance to heat.
The crude fatty acids used for the production of industrial fatty acid products are in most cases highly contaminated waste products from other industrial sources. Hydrogenation of these fatty acids is mainly carried out batchwise at temperatures from 100.degree. to 300.degree. and elevated hydrogen pressure of from 0.15 to 3.5 MPa in the presence of a suitable catalyst. Nickel catalysts in the form of finely divided nickel are mainly used for this purpose, either directly or deposited on silicon oxide as a carrier (Ullmann's Encyclopedia of Industrial Chemistry, vol. A10, page 268, VCH Weinheim 1987).
Disadvantages of this process are the high catalyst consumption due to the catalyst poisons present in the crude fatty acids (compounds of sulphur, phosphorus, chlorine and nitrogen), the formation of nickel soaps by the fatty acids and the need to remove the catalysts introduced into the crude fatty acids from the fatty acids by filtration after hardening. In addition, the product must be distilled to remove the nickel soaps.
The life of the catalysts can be increased and hence the consumption of catalysts reduced by distilling the crude fatty acids prior to hardening. This, however, requires the introduction of an additional process step and thereby generally renders the whole process less economical. One important criterion for assessing the quality of a catalyst for hardening is therefore its service life in operation when used for crude fatty acids, i.e., its resistance to catalyst poisons.
In addition to nickel catalysts, noble metal carrier catalysts are known for hardening fatty acids. Their advantages lie in the simplicity of the process and the absence of saponification. They are, however, easily damaged by the catalyst poisons present in the crude fatty acids (see, for example, M. Zajcew: "Fett-Hydrierung mit Palladium als Katalysator VII: Fettsauren" in Fett-Seifen-Anstrichmittel, 66 (1964), 794). Although noble metal carrier catalysts can be regenerated relatively easily with only slight loss of noble metal, these catalysts are used mainly for hardening distilled fatty acids in order to keep the catalyst poisoning low.
Noble metal carrier catalysts are particularly suitable for the continuous hardening of fatty acids. The precondition for this, however, is that not only the active noble metal component but also the carrier material should be sufficiently resistant to attack by the fatty acids. In the case of continuous hardening, the catalyst activity is defined by the so-called LHSV-value (Liquid hourly space velocity). The describes how much liquid volume can be converted in the desired manner per hour for a given volume of catalyst under the given process conditions (temperature, H.sub.2 -pressure).
A palladium catalyst on active charcoal is proposed in Canadian Patent Specification CA 1 157 844 for the continuous hardening of fatty acids. According to Example 7 of the said Patent Specification, this catalyst is capable, at LHSV-values of 0.2 h.sup.-1, of hardening distilled beef tallow fatty acids having an iodine number of 58 to final iodine numbers of from 0.2 to 0.7. The temperature employed was 190.degree. C. and the hydrogen pressure 2.5 MPa. When the LHSV-value was increased to 0.6 h.sup.-1 (Example 8), the final iodine number rose to 2.5-3.3. It has been found in practice that although relatively good hardening results are obtained for distilled fatty acids at low LHSV-values (0.2 h.sup.-1), the results are considerably inferior when crude fatty acids are used. This catalyst is therefore not suitable for economical hardening of fatty acids on a large industrial scale. The LHSV-values obtainable are too low and the necessary distillation of the crude fatty acids constitutes an additional, cost intensive process step.
Apart from noble metal catalysts on active charcoal carriers, noble metal catalysts on oxidic carriers are also known. These, however, are only used for selective hardening of fatty acids in the food industry. Thus, for example, U.S. Pat. No. 4,479,902 describes a catalyst of platinum or palladium on a carrier of TiO.sub.2, Ta.sub.2 O.sub.5, V.sub.2 O.sub.5 or Nb.sub.2 O.sub.5 for the selective hardening of soya bean oil to iodine numbers of 100. This catalyst is not suitable for the complete hardening of fatty acids owing to its high selectivity.